cabal-constraints: Repeatable builds for cabalized Haskell projects.

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'cabal-constraints' provides repeatable builds for cabalized Haskell
projects by "freezing" the exact versions of the dependencies selected by
'cabal-install'. All build environments for the project, such as the test
or staging build environments, or other developers collaborating on the
project, will then use the same dependency versions.

It is designed to be used alongside 'cabal-install' sandboxes, in which
case isolated, repeatable builds can be achieved.

Using cabal-constraints

'cabal-constraints' should be run from the root directory of a cabalized
Haskell project and given the path to the setup-config file to use. It will
print out all dependencies of the project in a format suitable for use in a
cabal-install config file. For example, running 'cabal-constraints'
against itself produces the following:

A single mandatory argument must be provided which is the path to the
setup-config file to use. The file will be located under the dist
directory. Usually there will be a single setup-config file which can be
found by running find dist -name setup-config from the root directory of the
project. If your project has more than one such file, it is likely because
you have built it either with and without sandboxes or with multiple
sandboxes. You probably want the most recently modified file which can be
found with ls -tr $( find dist -name setup-config ) | tail -n1.

To use these constraints for reproducible builds, one should make use of the
new sandbox feature of cabal-install 1.18. The constraints can be redirected
to cabal.config and committed to your code repository. When the project is
built, the same set of dependency versions will be resolved by cabal-install
ensuring repeatable builds.

If cabal.config contains no other information the simplest solution is to
overwrite it:

$ cabal-constraints > cabal.config

If cabal.config contains information that needs to be preserved, the
following will replace the constraints section and everything following it
with the new constraints. If you ensure that the constraints section is the
last section of the file, all other information in it will be kept.